Deserter from a regiment for a short time (3)
I believe the answer is:
rat
'deserter' is the definition.
(both can mean a traitor)
'a regiment for a short time' is the wordplay.
'a regiment' becomes 'RA' (Royal Artillery).
'for' says to put letters next to each other.
'a short time' becomes 't' (abbreviation; 'short' asks for an abbreviation).
'ra'+'t'='RAT'
'from' acts as a link.
(Other definitions for rat that I've seen before include "Abandon" , "Contemptible person" , "Despicable person" , "Treacherous person" , "person doing shopping" .)